1. Field of the Invention
The present invention relates to a field effect transistor having a variable gate capacitance and an operation method of the field effect transistor, and in particular relates to techniques for reduction in power consumption and reading error of the field effect transistor.
2. Related Art
A CMR (colossal magnetoresistive) material having a perovskite structure changes its properties according to external effects. For example, when a voltage pulse of an appropriate field intensity is applied at least once to a construction in which a thin film or bulk film of Pr0.7Ca0.3MnO3 (hereafter simply referred to as “PCMO”) is sandwiched between two electrodes, the PCMO film changes its properties. The properties that can change according to external effects include an electrical resistance (resistivity) and a capacitance (relative permittivity). These properties show different changes depending on a polarity of the voltage pulse applied. Once changed, the properties are stably maintained even after the application of the voltage pulse ends.
Taking advantage of this feature, a device that uses a PCMO film as a variable capacitance film is conventionally developed (see U.S. Patent Application Publication No. US 2004/0065912 A1). FIG. 1 is a sectional view showing a construction of such a device that uses a PCMO film as a variable capacitance film. In the drawing, a device 3 is formed by disposing an electrode 302, a PCMO film 303, and an electrode 304 on a substrate 301 in this order. The electrodes 302 and 304 are connected respectively with wires 302a and 304a. The electrode 304 has a circular main surface with a radius of 0.4 mm, and the PCMO film 303 has a film thickness of 600 nm.
When a voltage pulse of 18 V or −18 V is applied to the PCMO film 303 via the wires 302a and 304a, the PCMO film 303 changes in relative permittivity and electrical resistance. FIGS. 2A and 2B are graphs showing how these properties of the PCMO film 303 change upon repeated application of the voltage pulse. In detail, FIG. 2A shows changes in relative permittivity, whereas FIG. 2B shows changes in electrical resistance. In FIGS. 2A and 2B, the vertical axes respectively represent the relative permittivity and the electrical resistance, and the horizontal axes both represent time.
As shown in FIG. 2A, the relative permittivity of the PCMO film 303 changes to 405 upon application of a voltage pulse of −18 V, and changes to 135 upon application of a voltage pulse of 18 V. Also, as shown in FIG. 2B, the electrical resistance of the PCMO film 303 changes to 3500 Ω upon application of a voltage pulse of −18 V, and changes to 200 Ω upon application of a voltage pulse of 18 V. Once changed, the relative permittivity of the PCMO film 303 is maintained stably for at least three years. Hence a nonvolatile memory can be realized by relating the high and low levels in relative permittivity of the PCMO film 303 to the two binary states 0 and 1.
To change a relative permittivity of a PCMO film, in general it is necessary to apply a voltage pulse of at least several V to the PCMO film, though this varies depending on film thickness. To detect the relative permittivity of the PCMO film, on the other hand, it is sufficient to apply a voltage pulse of about 0.1 V to the PCMO film so as to detect a current. Thus, in the case where the PCMO film is used as a nonvolatile memory, writing data requires largest power.
In the above device 3, an area of the main surface of the electrode 304 is about 0.5 mm2, and the electrical resistance of the PCMO film 303 in a low resistance state is about 200 Ω. Accordingly, when a voltage pulse of 18 V is applied, a current of 90 mA flows, and 1.6 W of power is consumed. If the area of the main surface of the electrode 304 is reduced to 0.64 μm2 (0.8 μm×0.8 μm), the electrical resistance of the PCMO film 303 in a low resistance state is 25 KΩ. This being the case, when a voltage pulse of 5 V is applied, a current of 200 μA flows, and the power consumption can be reduced to 1 mW (see Technical Digest of IEEE International Electron Device Meeting (2002), p. 193).
However, when compared with a volatile memory such as an SRAM (static random access memory) whose power consumption is about 1 μW, the device 3 using the PCMO film still has an extremely high power consumption, and is unsuitable for practical use.